When a mobile station is switched on, it first scans the radio interface in order to detect the presence of a network. After it receives a signal from a base station, the mobile station implements a location update, and thereby informs the network of its location and identity. After these operations, the mobile station may initiate a radio resource session by requesting channel access. A channel refers herein to means of unidirectional transmission of information between at least two endpoints, herein a mobile station and a base station.
Access may be triggered in response to a communication need originating from the mobile station (a mobile originated call, location updating), or from the infrastructure side (mobile terminating call). Either way, the mobile station initiates the access procedures by sending an access request message to the base station. The base station decodes the received access request message, and after a successful decoding, forwards the access request to the network and thereby initiates a channel access procedure. The procedure comprises choosing of a free traffic channel, activating this channel in the base station and sending an assignment message to the mobile station. Once the mobile station has received the assignment message, it modifies its reception and transmission configuration to adapt to the frequency and time characteristics of the new channel, and starts establishing a link layer connection for signalling messages related to the original communication need.
The radio interface is a complex environment and provides various sources of interference, for example thermodynamic type of noise, and band noise from network or environment. Occasionally, the base station may misinterpret an invalid access request as a genuine one, and initiate unnecessarily the channel assignment procedure. Misinterpretation of bursts is also probable when mobile station at the limit of its sensitivity.
Such invalid channel access requests do not lead to a successful channel assignment establishment, and they should, in principle, be detected as early as possible, preferably already at the base station. It is clear that the processing of invalid channel access requests waste channel capacity, and thus degrade the network performance. However, from an operator's perspective, an even more disadvantageous effect is that the unsuccessful channel assignment procedures due to the undetected invalid channel access requests will show in Key Performance Indicator (KPI) figures of the operator as a failed call set-up. Since the KPI is an important driver of operator's actions, this may lead to unnecessary network optimization, testing and hours wasted in problem solving. In the worst case, the problem may degrade the accuracy of network coverage dimensioning.
Base transceiver stations may have internal design requirements that allow only a given number of invalid channel access requests in a certain period of time. Therefore detections made by the BTS should be as effective as possible. On the other hand, if the filtering of invalid channel access requests is too strict, genuine channel access requests may be inadvertently rejected, which degrades the receiver performance. Consequently, extreme care is required when measures for detecting and filtering out invalid channel access requests are designed.
A prior art method of detecting invalid channel access requests, known from networks implementing Global System for Mobile Communications (GSM), uses a two-staged procedure, which is based on determinations of two parameters:    1. Energy of estimated impulse response taps, where indications for channel access requests with inappropriate energies are filtered out. This stage aims to ensure that pure noise is not entered into the system.    2. Signal-to-noise ratio (SNR), where indications for channel access requests whose SNR does not exceed the threshold are filtered out. SNR plays a dominant role in detection of invalid channel access requests.
The procedures after the time the channel access request arrives at the base station and before the indication of the channel access request from the base station are herein referred to as receiver pre-processing. The procedures of receiver pre-processing may comprise, for example, timing offset correction, whitening, noise estimates, automatic frequency correction, etc. Depending on the implementation, the detection of invalid channel access requests is integrated to the pre-processing procedure. Presently, the problem with the current solution is that determination of SNR is measured at a defined stage of the receiver pre-processing and therefore some of the preceding functions, like the noise whitening, may affect the levels of the measured SNR.
A new SNR threshold are typically derived as a result of extensive simulations and analyses of measurement data collected by means of an actual product, which involves considerable amount of research and development (R&D) resources. However, due to varying customer needs or R&D specifications, the enhancements and alterations to the receiver features are unavoidable. For example, compensation of link imbalances and performance issues associated with new mobile station features typically require receiver improvements. Due to this interdependency between detection of invalid channel access requests and the pre-processing procedure, laborious adjustments are evident after any minor changes in the receiver configuration. This significantly reduces the flexibility of design and increases the R&D effort associated with releasing new products, both mobile stations and base station elements, to the market.